1. Field of the Invention
The present invention relates to a solid-state imaging device and a designing method thereof. More particularly, the present invention relates to a charged coupled device (CCD) or complementary metal oxide semiconductor (CMOS) image sensor which is incorporated in, e.g., a camera-equipped mobile phone.
2. Description of the Related Art
In, e.g., a camera-equipped mobile phone, a CCD or CMOS image sensor has been conventionally extensively used as a solid-stage imaging device. In such an image sensor, a condensing microlens is provided on a light incidence plane side of a photodiode in each pixel to improve a light detection efficiency.
Usually, an angle of light which exits from a camera lens (an imaging optical system) and enters a photodiode of each pixel differs between the center and a peripheral portion of an image area. Therefore, the larger the image area height, or the further the microlenses are positioned from the center of the image area, the more positions of them are shifted toward the center of the image area from the center of their corresponding photodiodes. This achieves uniformed optical sensitivities (light condensing efficiencies) among pixels.
For, example, for a camera lens having lens characteristics conforming to paraxial beam approximation, arrangement pitches of microlenses are set to be uniformly smaller than those of photodiodes, and each microlens is laid out in such a manner that a shift in position of each microlens from a position of a corresponding photodiode gradually increases as an image height increases (the arrangement pitch of the photodiodes is fixed). That is, light enters the photodiodes in the peripheral portion of the image area at a slant. Therefore, a position of each microlens with respect to a corresponding photodiode is gradually offset toward the center of the image area in a region extending from the center toward the edge of the image area. Pixels laid out in this manner can improve the light condensing efficiencies in the photodiodes in the peripheral portions of the image area. As a result, shading in the peripheral portions of the image area can be corrected, whereby nearly the same light condensing efficiencies can be assured in substantially the entire image region.
Further, as a similar technology, there is an attempt to reduce the shading by setting the arrangement pitch of microlenses to be smaller than that of photodiodes and shifting a position of each microlens in a peripheral portion of an image area toward the center of the image area from the center of a corresponding photodiode (see, for example, Japanese Patent No. 2600250).
On the other hand, in regard to a camera lens having lens characteristics which do not conform to paraxial beam approximation, the following suggestions are already present.
For example, for a camera lens that an exit angle of a chief ray from the final plane of an imaging optical system does not uniformly monotonously increase with a rise in an image height from an optical axis, an arrangement pitch of microlenses in at least a part of a region extending from the center of an image area to a predetermined position in a peripheral portion of the same is reduced and the arrangement pitch of the microlenses in at least a part of the region of the peripheral portion beyond the predetermined position is increased to be larger than the counterpart in the aforesaid region. This can correct the shading (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-228645).
For example, for a camera lens that an exit angle of a chief ray from the final plane of an imaging optical system increases until a given image height from an optical axis is reached and decreases beyond this image height, an arrangement pitch of microlenses in a region with a large absolute value of an exit pupil position of the camera lens is decreased and the arrangement pitch of the microlenses in a region with a small absolute value of the exit pupil position is increased. This can assure nearly the same light condensing efficiencies in an entire image area (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-237150).
However, in a recent image sensor, the layout of interconnects including gate electrodes is becoming difficult with miniaturization of pixels, it is unavoidable that the interconnects block a part of incident light to lose a part of light entering photodiodes. For example, for an image sensor with a pixel pitch (an arrangement pitch of photodiodes) of 1.4 μm, assuming that one interconnect with width of 0.2 μm lies between photodiodes, a distance between the interconnect and each photodiode is 0.6 μm based on an expression (1.4 μm−0.2 μm)/2=0.6 μm. A wavelength of reflected light from a red subject is approximately 600 to 700 nm. Therefore, considering a wave-optical effect, an image sensor having a small pixel pitch must be designed on the assumption that a part of incident light is lost by interconnects.
That is, when laying out interconnects becomes difficult as miniaturization of pixels advances, the layout of pixels is horizontally or vertically asymmetric with respect to the center of each photodiode depending on the layout of the interconnects (note that layouts of the pixels are uniform in an image area). For example, it is assumed that a gate electrode is arranged on a right-hand side of a photodiode in each pixel. Then, the gate electrode partially covers the right-hand side of the photodiode. Therefore, the gate electrode blocks more incident lights in pixels in a left peripheral portion of an image area than in a right peripheral portion. That is, since incident light enters in different angles between in the left peripheral portion and the right peripheral portion of the image, the pixels in the left peripheral portion has more losses of the incident lights blocked by the gate electrodes. It is possible to provide the gate electrode (e.g., a dummy interconnect) on the left-hand side of the photodiode to likewise cover the left-hand side of the photodiode to equalize the blockade of the incident lights by the gate electrodes. This approach, however, is not suitable for miniaturization. Therefore, an image sensor that has increasing difficulties to secure a margin between interconnects due to miniaturized pixels faces an obstacle that blocks improving of the light condensing efficiencies in the entire image area unless a shift of each microlens with respect to a corresponding photodiode is controlled with the interconnects layout considered.